According to the prior art, tunable band-pass filters comprise resonator elements made of ferrites, in which the resonance frequency is adjusted via an external DC magnetic field. The resonators are generally spherical, because this shape can be manufactured using relatively-simple techniques with the dimensions required for use at high frequencies (diameter of sphere ≦0.3 mm). One reason for using spherical resonators is the linear relationship between the resonance frequency and the modulus of the external DC magnetic field.
Yttrium iron garnet (YIG) is used as the material for the resonators at frequencies up to approximately 50 GHz. For frequencies above 50 GHz, the use of hexaferrites has proved preferred. Because of their crystalline structure, hexaferrites provide an anisotropic field, which, with a corresponding orientation relative to the external DC magnetic field, allows the adjustment of high resonance frequencies with significantly-lower field strengths of the DC magnetic field than is possible when using YIG. This property of hexaferrites, allows an avoidance according to the prior art of the technically-demanding generation of high magnetic-field strengths for the adjustment of high resonance frequencies.
Shielded (suspended) striplines are disposed, for example, in channels milled entirely into metal. These channels are connected to one another exclusively via a circular coupling aperture (iris). The prior art assumes that the lines are disposed perpendicular to one another, which leads to high decoupling outside the resonance in view of the orthogonality of the electromagnetic fields. As in case of many other coupler structures according to the prior art, the spheres within the structure are attached in the proximity of a short-circuit. The reason for this is that the resonators, especially the resonator spheres, are coupled via the magnetic field (HF field), which is maximal in the region of the short circuit. Since, according to the prior art, this maximum occurs in the region of the short circuit independently of the frequency, a good coupling of the spheres is achieved over a large frequency range in resonant conditions.
Furthermore, by contrast with non-resonant conditions, field energy supplied through the ferrite properties of the spheres in resonant conditions is radiated in the direction of the diaphragm, thereby leading to an increased energy transfer between the filter input and the filter output.
One possibility according to the prior art for reducing the insertion loss of the filter under otherwise identical conditions (identical line width of the resonance curve of the resonator, identical saturation magnetization of the resonator and identical diameter of the iris) is the use of inverse shielded (suspended) striplines. With this type of line, the middle conductor is attached to the side of the substrate directed towards the resonator or respectively the resonator sphere, wherein the resonators continue to be disposed in the region of the short circuit and to provide the disadvantages associated with this.
In the context of the prior art, it is dispreferred if the magnetic field provides a considerable component parallel to the direction of transport of the decoupled wave in the short-circuited region of two metallic strips within the proximity of the coupling. As a result, disturbing auxiliary modes can be excited by the coupling.
U.S. Pat. No. 4,888,569 B1 specifies coupler structures with four resonator spheres for use in magnetically-tunable filters. By way of example, this patent discloses a variable band-pass filter for frequencies within a maximum frequency range of one waveguide band, for example, 50-75 GHz. The variable band-pass filter comprises an input waveguide, an output waveguide and a transition waveguide, which are designed for the propagation of a TE10 wave mode. During the operation of the filter, the end of the input waveguide terminated with a short-circuit wall, the beginning of the output waveguide, which is also provided with a short-circuit wall, and the transition waveguide attached in the direction towards the externally-applied, homogenous magnetic field below the input waveguide and the output waveguide, are arranged between two magnetic poles, which supply the variable magnetic field for the adjustment of a resonance frequency. The input waveguide and output waveguide provide a rectangular profile in the direction of the wave propagation, which provides a significantly-smaller cross-sectional area in the coupling region than at the connecting flange. The coupling region of the variable band-pass filter encloses the four resonator spheres attached in the proximity of a short-circuit wall and respectively the tapering end of the input waveguide and output waveguide, and the transition waveguide with a constant cross-sectional area.
One disadvantage of the variable band-pass filter described in U.S. Pat. No. 4,888,569 B1 is that in resonant conditions, the field distribution of the wave to be decoupled is unfavorable in the coupling region, because the wave is conducted in a waveguide, of which the profile tapers towards the coupling region in a direction perpendicular to the direction of propagation of the wave to be decoupled. As a result, undesirable reflections occur, which overlap in a destructive manner and therefore reduce the amount of energy transported by the incoming wave. This effect also relates to the outgoing wave in the output waveguide, which now provides a defined frequency. Accordingly, the overall insertion loss relative to the input of the input waveguide and the output of the output waveguide is increased, because the field distributions in the coupling region are disturbed by the tapering geometry of the waveguides.
One further disadvantage is the limited bandwidth of the waveguide concept.